WikiJournal of Science, 2018, 1(2):7 doi: 10.15347/wjs/2018.007 Encyclopedic Review Article Lead: properties, history, and applications Mikhail Boldyrev¹*, et al. Abstract Lead is a chemical element with the atomic number 82 and the symbol Pb (from the Latin plumbum). It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and has a relatively low melting point. When freshly cut, lead is silvery with a hint of blue; it tarnishes to a dull gray color when exposed to air. Lead has the highest atomic number of any stable element and concludes three major decay chains of heavier elements. Lead is a relatively unreactive post-transition metal. Its weak metallic character is illustrated by its amphoteric nature; lead and its oxides react with acids and bases, and it tends to form covalent bonds. Compounds of lead are usually found in the +2 oxidation state rather than the +4 state common with lighter members of the carbon group. Exceptions are mostly limited to organolead compounds. Like the lighter members of the group, lead tends to bond with itself; it can form chains, rings and polyhedral structures. Lead is easily extracted from its ores; prehistoric people in Western Asia knew of it. Galena, a principal ore of lead, often bears silver, interest in which helped initiate widespread extraction and use of lead in ancient Rome. Lead production declined after the fall of Rome and did not reach comparable levels until the Industrial Revolution. In 2014, annual global production of lead was about ten million tonnes, over half of which was from recycling. Lead's high density, low melting point, ductility, and relative inertness to oxidation make it useful. These properties, com- bined with its relative abundance and low cost, resulted in its extensive use in construction, plumbing, batteries, bullets and shot, weights, solders, pewters, fusible alloys, white paints, leaded gasoline, and radiation shielding. In the late 19th century, lead's toxicity was recognized, and its use has since been phased out of many applications. Lead is a toxin that accumulates in soft tissues and bones, it acts as a neurotoxin damaging the nervous system and interferences with the function of biological enzymes. It is particularly problematic in children: even if blood levels are promptly normalized with treatment, neurological disorders, such as brain damage and behavioral prob- lems, may result Physical properties radii from lanthanum (atomic number 57) to lutetium (71), and the relatively small radii of the elements after hafnium (72). This is due to poor shielding of the nucleus Atomic by the lanthanide 4f electrons. The combined first four A lead atom has 82 electrons, arranged in an electron ionization energies of lead exceed those of configuration of [Xe]4f145d106s26p2. The combined first tin,[1]contrary to what periodic trends would predict. and second ionization energies—the total energy re- Relativistic effects, which become significant in heavier quired to remove the two 6p electrons—is close to that atoms, contribute to this behavior.[a] One such effect is of tin, lead's upper neighbor in the carbon group. This is the inert pair effect: the 6s electrons of lead become re- unusual; ionization energies generally fall going down a luctant to participate in bonding, which leads to ele- group, as an element's outer electrons become more vated ionization energies and makes the distance be- distant from the nucleus, and more shielded by smaller tween nearest atoms in crystalline lead unusually orbitals. The similarity of ionization energies is caused long.[3] by the lanthanide contraction—the decrease in element Lead's lighter carbon group congeners form stable or metastable allotropes with the tetrahedrally coordi- nated and covalently bonded diamond cubic structure. 1 Department of Optimal Control, Faculty of Computational Mathe- matics and Cybernetics, Moscow State University, Moscow, Russia The energy levels of their outer s- and p-orbitals are 3 *Author correspondence: [email protected] close enough to allow mixing into four hybrid sp orbit- ORCID: [0000-0000-0000-0001] als. In lead, the inert pair effect increases the separation Licensed under: CC-BY-SA between its s- and p-orbitals, and the gap cannot be Received 22-11-2017; accepted 03-07-2018 1 of 23 | WikiJournal of Science WikiJournal of Science, 2018, 1(2):7 doi: 10.15347/wjs/2018.007 Encyclopedic Review Article overcome by the energy that would be released by ex- The melting point of lead—at 327.5 °C (621.5 °F)[24]—is tra bonds following hybridization.[4] Rather than having very low compared to most metals.[15][g] Its boiling point a diamond cubic structure, lead forms metallic bonds in of 1749 °C (3180 °F)[24] is the lowest among the carbon which only the p-electrons are delocalized and shared group elements. The electrical resistivity of lead at 20 between the Pb2+ ions. Lead consequently has a face- °C is 192 nanoohm-meters, almost an order of magni- centered cubic structure[5] like the similarly sized[6] diva- tude higher than those of other industrial metals (cop- lent metals calcium and strontium.[7][b][c][d] per at 15.43 nΩ·m; gold 20.51 nΩ·m; and aluminium at 24.15 nΩ·m).[26] Lead is a superconductor at tempera- Bulk tures lower than 7.19 K;[27] this is the highest critical temperature of all type-I superconductors and the third Pure lead has a bright, silvery appearance with a hint of highest of the elemental superconductors.[28] blue.[12] It tarnishes on contact with moist air, and takes on a dull appearance the hue of which depends on the Isotopes prevailing conditions. Characteristic properties of lead include high density, malleability, ductility,[e] and high Natural lead consists of four stable isotopes with mass resistance to corrosion due to passivation.[13] numbers of 204, 206, 207, and 208,[29] and traces of five short-lived radioisotopes.[30] The main isotopes of lead, Lead's close-packed face-centered cubic structure and with information on percent abundance, half-life, and high atomic weight result in a density[15] of 11.34 g/cm3, decay mode and product, are listed in Table 1. The high which is greater than that of common metals such as number of isotopes is consistent with lead's atomic iron (7.87 g/cm3), copper(8.93 g/cm3), and zinc (7.14 number being even.[h] Lead has a magic number of pro- g/cm3).[16] This density is the origin of the idiom to go tons (82), for which the nuclear shell model accurately over like a lead balloon.[17][18][f] Some rarer metals are predicts an especially stable nucleus.[31] Lead-208 has denser: tungsten and gold are both at 19.3 g/cm3, and 126 neutrons, another magic number, which may ex- osmium—the densest metal known—has a density of plain why lead-208 is extraordinarily stable.[31] 22.59 g/cm3, almost twice that of lead.[19] With its high atomic number, lead is the heaviest ele- Lead is a very soft metal with a Mohs hardness of 1.5; it ment whose natural isotopes are regarded as stable; can be scratched with a fingernail.[20] It is very malleable lead-208 is the heaviest stable nucleus. This title was and quite ductile.[21] The bulk modulus of lead—a meas- formerly held by bismuth, with an atomic number of 83, ure of its ease of compressibility—is 45.8 GPa. In com- until its only primordial isotope, bismuth-209, was parison, that of aluminium is 75.2 GPa; copper 137.8 found in 2003 to decay very slowly.[i] The four stable iso- GPa; and mild steel 160–169 GPa.[22] Lead's tensile topes of lead could theoretically undergo alpha decay strength, at 12–17 MPa, is low (that of aluminium is 6 to isotopes of mercurywith a release of energy, but this times higher, copper 10 times, and mild steel 15 times has not been observed for any of them; their predicted higher); it can be strengthened by adding small half-lives range from 1035 to 10189 years.[34] amounts of copper or antimony.[23] Three of the stable isotopes are found in three of the four major decay chains: lead-206, lead-207, and lead- 208 are the final decay products of uranium-238, ura- nium-235, and thorium-232, respectively. These decay Isotope Decay abundance half-life (t1/2) mode product 202 4 202 Pb syn 5.25(28)×10 y ε Tl 204 Pb 1.4% stable 205 7 205 Pb trace 1.53(7)×10 y ε Tl 206Pb 24.1% stable 207Pb 22.1% stable 208Pb 52.4% stable 209 − 209 Pb trace 3.253(14) h β Bi 210 − 210 Pb trace 22.3(22) y β Bi 211 − 211 Pb trace 36.1(2) min β Bi 212 − 212 Pb trace 10.64(1) h β Bi 214 − 214 Pb trace 26.8(9) min β Bi Figure 1 | A sample of lead solidified from the molten state. Table 1 | Main isotopes of lead. Isotopic abundances vary Juergen Kummer, CC-BY 3.0 greatly by sample. Standard atomic weight: 207.2(1)[14] (Ar, standard) 2 of 23 | WikiJournal of Science WikiJournal of Science, 2018, 1(2):7 doi: 10.15347/wjs/2018.007 Encyclopedic Review Article samples by measuring its ratio to lead-206 (both iso- topes are present in a single decay chain).[42] In total, 43 lead isotopes have been synthesized, with mass numbers 178–220.[29] Lead-205 is the most stable radioisotope, with a half-life of around 1.5×107 years.[j] The second-most stable is lead-202, which has a half- life of about 53,000 years, longer than any of the natural trace radioisotopes.[29] Chemistry Bulk lead exposed to moist air forms a protective layer Figure 2 | The Holsinger meteorite, the largest piece of the of varying composition.